The present invention relates to a polyurethane elastomeric fiber made by melt spinning a urethane having a rebound of 60% or higher and a isocyanate-terminated polyether or polyester to yield elastic fibers with low shrinkage, high heat resistance, low tensile sets and high elongation. Polyurethanes having improved elastomeric properties have many practical applications such as fabrics in consumer goods, such as hosiery and clothing, medical applications, recreational applications, automotive applications, or industrial applications, such as conveyor belting, cable jacketing, and the like.
A common elastic fiber used in the clothing industry is spandex. It is a stretchable fiber with a high elastic recovery. Spandex is used in many commercial yarns and fabrics to add elasticity to the clothing.
Spandex is defined by the Federal Trade Commission as a long-chain synthetic polymer comprising at least 85% of a segmented polyurethane. It is made by reacting a high molecular weight dihydroxy compound with an organic diisocyanate and chain extender to form an elastomer polymer. It is segmented because it is composed of alternating soft and hard regions within the polymer structure. The hard segments act as physical cross-links that tie the polymer chains together. The soft segments of the polymer chains are composed of polymers containing long, coiled, segments that can orient upon stretching the material. The cross-links prevent the polymer chains from moving significantly past each other. After stretching, the linear soft segments recover to a coiled form so that the fiber returns to its original shape.
It is preferably made by a dry spun process, although wet spinning and melt spinning processes are known. Du Pont first introduced spandex elastic polyurethane(urea) fiber in 1959, using a dry-spinning method. In 1937 Bayer introduced polyurethane elastic fiber made by a melt spinning method.
The dry spinning method is a process in which a long chain diol is reacted with aromatic diisocyanate (usually 4,4′-methylene diphenyldiisocyanate, MDI) to produce an isocyanate-terminated prepolymer. Chain extension is accomplished by reacting the prepolymer with diamine in the presence of a polar solvent to prepare spinning dope. The dope is then extruded from the nozzle in multi-filament form. The solvent is evaporated by coming in contact with hot air or hot N2 in the spinning column. The spun yarn is then false-twisted, oil-treated and wound up on a bobbin. Dry spun fibers tend to lose strength upon aging.
The melt-spinning method is different in that both prepolymer preparation and chain-extension are conducted in the absence of solvent. To achieve a fiber with properties comparable to those achieved by dry spinning, it is necessary to subject the spun fiber to heat-aging treatment to promote cross-linking through the remaining isocyanate group.
The melt spinning method for polyurethane thermoplastic elastomers developed by Bayer in the late 1930s and early 1940s did not give satisfactory properties. Other melt spinning processes for polyurethanes are known. U.S. Pat. No. 3,503,933 disclosed a melt spun spandex which used asymmetric diisocyanates which have a five-fold difference in the reactivity of the two isocyanate groups. These diisocyanates resulted in the spandex having poor processing qualities such as high tack, poor fabric qualities such as low unload power, poor fabric processing properties in areas such as dyeing, finishing, and laundering. U.S. Pat. No. 5,840,233 to Foss et al. teaches a process for making melt-spun elastomeric fibers from a melt-spinnable elastomeric polymer comprising a diisocyanate-capped polyol prepolymer that is chain-extended with an aromatic dihydroxy compound. The fibers produced can be knit or woven into textile articles, such as hosiery or pantyhose.
U.S. Pat. No. 6,127,506, Green teaches a process for melt spinning spandex in which a polyurethane(urea) polymer is prepared from a purified capped glycol, linear aliphatic diamines and a monoamine chain terminator. During the process, the diisocyanate is contacted with the polymeric glycols to yield a capped glycol. The process in Green focuses on prepurifying the capped glycol prior to the formation of the polyurethaneurea. The polymeric glycols used in the process include polyether glycols, polyester glycols, polycarbonate glycols, and copolymers of the glycols. The diisocyanate has substantially the same reactivities toward the hydroxyl groups as the polymeric glycol. The preferred diisocyanate is MDI. In Green, the monomers are polymerized and then melt spun to yield spandex or polyurethane(urea).
Traditionally, spandex has been prepared by either dry-spinning or wet-spinning. Melt spinning is the most advantageous of the processes in terms of health, safety and environmental concerns because it does not involve the use of organic solvents. But, the dry-spun spandex produces a fiber having the best balance of properties compared to those made by the other processes. Therefore, a fiber material with the properties of spandex but which can be melt-spun instead of dry-spun is desirable. Although thermoplastic polyurethanes generally have desirable properties with regard to abrasion resistance, they do not generally exhibit properties such as high melting point, low tensile set, low compression set, good rebound, and low hysteresis.
Polyether polyurethanes are known. For example, U.S. Pat. No. 5,959,059 to Vedula et al. teaches a polyether urethane that has good physical properties when prepared by the melt polymerization of a hydroxyl-terminated polyether intermediate, containing alkylene oxide repeat groups of from 2 to 6 carbon atoms, and a chain extender with a diisocyanate. Vedula et. al, used aromatic diisocyanates, with MDI being preferred. Further, the thermoplastic polyether urethanes were produced by a “one-shot process,” where the hydroxyl terminated polyether intermediate, the chain extender, and the diisocyanate were added together, mixed, and polymerized. Polyurethanes produced by this “one-shot” method are polyurethanes in which the chain extender includes aromatic moieties, having higher melting points. The resulting polyether urethanes exhibited high melting points, from 170° C. to 230° C., low densities, 1.10 or less, and Shore D hardness of at least 15 or 20. Additionally, the polyether urethanes exhibited good tear resistance, good abrasion resistance, and hydrolytic stability.
Melt spinning of polyurethane polymers is also known, including combining, with the polyurethane polymer in the melt additional materials to achieve various benefits. For example, Japanese Patent Publication JP58098421 (1983) (Yasuhiro et al.) teaches adding a reaction product from a polyisocyanate and a blocking agent to a polyurethane elastomer to produce an elastic yarn with high recovery from deformation at high temperatures. The reaction in the Yasuhiro publication, is between a polyisocyanate of 400 or more molecular weight, preferably p,p-diphenyl methane diisocyanate (MDI) and polytetramethylene glycol on both terminals and a blocking agent. Japanese patent publication 60048617 (2000) to Yamakawa et al., teaches melt spinning butylene terephthalate-based crystalline polyester and thermoplastic polyurethane to make an elastic fiber having a degree of luster of 70 or less. Japanese patent publication No. JP1282387 (1989) to Yoshimoto et al. teaches an elastic polyurethane yarn produced by kneading a polyisocyanate prepolymer as a crosslinking agent, where the yarn is subsequently treated with a mineral oil, polysiloxane, and diamine mixture, to prevent the sticking of the fibers. Japanese patent publication No. JP58186609 (1983) to Ogawa teaches fibers having improved heat resistance made by adding a polyisocyanate compound and a pigment to a molten polyurethane elastomer. Japanese patent publication No. JP57112409 (1982) to Ogawa et al. teaches an elastic yarn of high recovery from the deformation at elevated temperature made by adding to a molten polyurethane, a polyisocyanate reaction product having blocked NCO terminals from a polyether of 300 to 2,500 molecular weight and diphenylmethane diisocyanate.